1. Field of the Invention
The invention relates in general to a light emitting device display technique, and more particularly, to an organic light emitting diode (OLED) for reducing power source resistance. Thereby, a stable and uniform driving voltage is supplied to each pixel of the organic light emitting diode.
2. Related Art of the Invention
Following with the advancement in high-tech, video products, particularly digital video products or image apparatus have become one of the most common products in our daily lives. Among various video products or image apparatus, the display is an important device for information display. The user can read information from a display, or even control the operation of an apparatus.
To compliment the modern life style, the video products or image apparatus are developed thinner and lighter. The traditional cathode ray tube display, though having certain advantages, has been replaced by flat panel display fabricated by optoelectronic and semiconductor process due to the large volume and power consumption. The commonly seen flat panel display includes liquid crystal display or active organic matrix light emitting diode, for example.
The technique of liquid crystal display has been developed for years and a breakthrough is hard to attain. The organic light emitting diode technique has thus become the main stream of modern flat panel display. The characteristics of the organic light emitting diode includes forming the thin-film transistor (TFT) driving technique to drive the light emitting diode, and forming the driving IC on the panel directly to achieve the objectives of being light, thin, short and small, and low-power consumption. The organic light emitting diode can be applied to the small and medium sized panels of cellular phones, personal data assistant (PDA), digital camera, game boy, portable DVD player and vehicle navigation systems. The application of the organic light emitting diode may further include larger area flat panel displays such as computer and television screens.
The digitalized display is characterized in constructing the display screen with a plurality of pixels arranged as an array. To control each individual pixel unit, a scan line and a data line are formed to select the specific pixel and to provide appropriate operating voltage thereto, so as to display information corresponding to the selected pixel.
FIG. 1 shows a schematic diagram of a circuit for driving the pixel array of an organic light emitting diode. The display can normally be categorized into an active type and a passive type. The following provides description of a passive circuit. Referring to FIG. 1, the scan line is connected to the gate of each thin-film transistor 100 for each pixel. Within a frame time, the scan line is switched on. The data line is connected to a source/drain region of each thin-film transistor 100 to supply a voltage to the corresponding pixel for data input. The voltage is maintained by the capacitor 110, and the thin-film transistor 104 is switched on. Meanwhile, the source line 116 converts a power source Vdd into a current via a source current source thin-film transistor 104. The current flows through the light emitting diode 108 for generating light. Therefore, the voltage of a data line determines the light, including the gray scale thereof, emitted from the pixel.
An image is typically constructed by a plurality of pixels arranged as a two-dimensional array, for example, 640×480 pixels. In other words, one source line 116 may provide a voltage to all the light emitting diodes 108 in a row or a column. Although the source line 116 is made of a conductive material such as polysilicon, the source line 116 still has certain resistance. Therefore, for a source line 116 with long extension, the voltage applied to the first pixel is larger than that applied to the last pixel. As the area of the panel increases, this voltage difference becomes more significant. FIG. 2 shows the structure of a conventional source line. The source line 116 extends across the scan line 112 and is connected to the thin-film transistor via the plug 120. The scan line 112 and the source line 116 are isolated from each other by an insulation layer (not shown). The source line 116 is typically made of polysilicon. The insulation layer includes a conductive plug 120 coupled with the source line 116. When a driving voltage Vdd is applied across the source line 116, a voltage drop is typically caused by the resistance of the source line 116. The source line 116 may be formed of material with high conductivity. However, the process may be more complex, and the cost is increased. As polysilicon material can be defined together with other relative devices of the thin-film transistor, so that not a lot of additional process is required by forming the polysilicon source line. However, the conductivity of polysilicon source line with long extension may cause voltage drop due to the internal resistance thereof.
Therefore, the brightness of the light emitting diode 108 is often varied according to the driving voltage Vdd. In the above conventional design, the uniformity of the panel brightness is always a challenge. As the driving method for organic light emitting diode requires a current source thin-film transistor 104 to convert the driving voltage into a current, the stability of the voltage directly affects the current flowing to the organic light emitting. Particularly, following the trend of increasing panel dimension, the pixel line is longer, and the line resistance is larger causing non-uniform brightness.